1. Field of the Invention
The present invention relates to ground station equipment for use with Differential Global Positioning Systems (DGPS) which utilize signals from a plurality of satellites to determine various parameters of aircraft operation such as position, attitude, velocity and the rates of change of these parameters and which provide correction information for satellite specific pseudo range errors.
2. Description of the Prior Art
GPS systems are well-known in the prior art. Examples of such prior are systems are U.S. Pat. No. 5,361,212 of the present assignee and 4,894,655 assigned to LMT Radiopreofessionnelle, of Boulogne Billancourt, France.
For simplicity a simple prior art system is shown herein in FIG. 1 in which an aircraft 10 is shown receiving signals from four remote satellites (FS1, FS2, FS3, and FS4) over paths shown by arrows 12, 14, 16 and 18 respectively. GPS receiver equipment aboard aircraft 10 can operate on the signals to determine the desired aircraft parameters (position, speed, attitude and the rates of change parameters etc.) useful in the navigation thereof. It is known, however, that because of various factors such as tropospheric and ionispheric distortions, the signals from the satellites may contain errors herein referred to as "satellite-specific pseudo range errors" which errors cause inaccuracies in the determination of the aircraft parameters. Accordingly, more advanced prior art systems have utilized DGPS receivers shown in FIG. 1 as box 20 having an antenna 22 located at a fixed position which is known with great accuracy. DGPS receiver 20 is shown as having a receiving antenna 22 which receives information from the satellites that are within it's view and, in the present case, from the four satellites FS1, FS2, FS3 and FS4 over paths shown as arrows 24, 26, 28 and 30. While a single antenna is shown in FIG. 1, more than one and preferably three antennas may be used so that multipath distortions caused by nearby objects (building, trees etc.) can be minimized. DGPS receiver 20 calculates the ranges as determined from the satellites signals and sends this information via a connection shown as arrow 32 to a microprocessor 34 which, knowing the actual position of the DGPS receiver, determines the satellite-specific pseudo range errors and produces information signals of these errors over a line shown as arrow 36 to a transmitter 38 having a transmitting antenna 40. Antenna 40 transmits the satellite-specific pseudo range error information to the aircraft 10 as shown by arrow 42 and also to a locally fixed antenna 44 connected to a receiver 46 which produces a signal back to the ground station 20 over the line shown as arrow 48 indicative of what was sent to aircraft 10. This feedback signal is referred to as a data link wraparound and is for the purpose of informing the ground station of the exact signal that was sent to the aircraft 10 as a check to make sure that the ground station system comprised of 22, 20, 32, 34, 36, 38, 40 is maintaining accuracy.
Aircraft 10 can thereafter utilize the now known error information to modify it's own calculations of the aircraft parameters so that it is assured that it's calculations are accurate.
Systems such as shown in FIG. 1 may be used near or at various airports around the world to direct aircraft in for landings and, after landing, to direct aircraft in a taxi mode over the ground paths to a terminal. This signal is useful for aircraft in a region within a 100 mile circle with the center located at the antenna 22.
Difficulties have been encountered with respect to ground station systems as shown in FIG. 1 because: 1) antenna 40 may not be able to see all portions of an aircraft landing site due to obstructions from buildings, natural surroundings and 2) when several airports are located in relatively close proximity to each other, the signals being transmitted to the aircraft cannot cover the entire region and 3) the current landing system (ILS) uses numerous frequencies in their band causing frequency congestion.
To solve this latter problem the FAA in a May 10, 1994 revision of Appendix F of a document DO-217 has defined requirements for the transmission of DGPS messages utilizing RTCA time slot allocation as shown in FIG. 2. In FIG. 2, three periods of transmission are shown as T1, T2 and T3 respectively and these are divided into a plurality, for example, 8 sub-time slots as identified by the Arabic numerals 1-8 in FIG. 2. FIG. 2 assumes that there are three different transmitters TX1, TX2, and TX3 that are sending information to an aircraft from locations which could overlap so that the aircraft could receive the transmissions simultaneously and, if they were at the same frequency, might possibly produce confusion. In order to use a single frequency, therefore, the FAA has proposed that the first transmitter TX1 transmit it's information only during the sub-time slot 1 in each period of transmission while transmitter TX2 transmits it's information to the aircraft only during sub-time slot 2 of each transmission period and transmitter TX3 transmits it's information only during the sub-time slot 3 of each transmission period. Obviously with eight sub-periods five more transmitters could be utilized in this system with all of them broadcasting on the same frequency with the aircraft avoiding confusion by knowing which transmitter is using which sub-time slot. To use the prior art systems in a plurality of locations requires a great deal of duplication of equipment.